The present invention relates to a high pressure fuel injection system with a common rail design in which the rail internal volume is selectable between a smaller volume which may be used, for example, during engine starting and a larger volume for steady state operation beneficially characterized by lessened pressure fluctuation.
Designers of common rail fuel injection systems for internal combustion engines must compromise between low internal volume to allow rapid engine starting and high rail volume which serves as a capacitance to damp out pressure waves that are induced when the injector flows start and stop.
The engine must fire within a fraction of a second from the time of key on to be driver acceptable. In this short period of time, the high pressure fuel pump must bring the rail pressure sufficiently high to support reasonable combustion, i.e., robust, avoiding high emissions, and generating sufficient power to start up the engine. Measures taken to ensure rapid starts include limiting the internal volume of the rail and selecting a pump design with sufficient pumping capacity. The latter measure leads to the pump to be of higher capacity than otherwise needed and in turn higher frictional losses.
Another constraint on the common rail is the need to damp reflecting pressure waves that are caused by the intermittent flow through the fuel injectors. Not only do pressure waves exist due to each injector""s actions separately, but the individual pressure waves are summed in the common rail and may reinforce each other or resonate under some operating conditions. The problem which pressure waves cause is uneven metering. The amount of fuel which is injected is a function of pulse width, pressure drop across the fuel nozzle, the cross-sectional area of the nozzle orifice, the number of orifices, and the discharge coefficient across the orifice. If the pressure in the fuel line is fluctuating, particularly on the time scale of an injection event (order of a millisecond) fuel metering becomes complicated. To alleviate this difficulty, measures taken include maximizing the internal volume of the rail and selecting an internal geometry which attenuates the pressure waves. Large internal volume of the rail attenuates pressure waves due to the fact that liquid fuels, although normally thought of as incompressible, are quite compressible at the pressures existing in diesel fuel injection systems (order of 1800 bar). The larger the internal volume of the rail, the greater the capacitance and the desired damping is achieved. However, as mentioned above, increases in internal volume negatively affect engine starting, i.e., there is a constraint on how far internal volume can be increased. Another measure taken to suppress pressure waves is to design the internal space of the rail to contain a spherical chamber in which the isotropy of the inside surface tends to confound the pressure waves, i.e., bouncing unpreferentially in all directions such that reinforcement of the individual waves is lessened.
From the above discussion, the desired fuel rail internal volume must selectively be small or large to satisfy conflicting demands. In actuality, a fuel rail volume which satisfactorily bridges the compromise does not exist. Thus, additional measures are taken to facilitate the quick starting and minimal pressure fluctuation desired. If the fuel rail could be as small as desirable for rapid starting, the demands on the fuel pump would be lessened; a pump with fewer and/or smaller plungers might be employed. This would measurably improve cold start fuel consumption and directionally improve fuel consumption over the remainder of the operating map. Another potential advantage would be to change pump drive ratio, which would allow the use of a smaller pump operating at higher speed. To satisfy the need to dampen pressure fluctuations, a large internal rail volume is desired but cannot be achieved without harming starting times. A measure helping to reduce pressure fluctuations within the rail is a spherical internal rail geometry. The disadvantage is that this construction is more expensive than a cylindrical internal geometry.
The subject of the invention disclosed herein is a high pressure fuel injection system for an engine which includes both a small and a large volume rail. The small volume rail can be isolated from the large volume rail when necessary for engine start. By having the rail volume selectable between the smaller and larger volumes, both rapid start and low pressure fluctuation in the common rail during normal operation can be attained without compromising the system.
After the ignition key is turned on, the engine""s control system selects the appropriate valve positions so that the large volume rail is not in communication with the small volume rail. As the high pressure fuel pump raises pressure in the small rail to a satisfactory pressure for robust operation, a variable valve located between the small and large volume rails and controlled by the engine controller is partially opened. As used herein, the term variable valve means pulse width modulated, variable opening, or other type of valve known to those skilled in the art and suggested by this disclosure.
Opening of the variable valve causes the pressure in the large volume rail to rise. After the large volume rail pressure attains an operating level, a valve located in between the high pressure pump and the large volume rail and the variable valve are opened fully. At this point steady-state, normal engine operation has been achieved.
A high pressure fuel injection system for a reciprocating internal combustion engine having at least two cylinders includes a fuel supply line connected to a high pressure pump, a fuel line containing a valve connecting the high pressure fuel pump to a large volume rail, a fuel line connecting the high pressure fuel pump to a small volume rail, a fuel line containing a variable valve connecting the large volume and small volume rails with a variable valve, and fuel lines from the small volume rail to the fuel injectors. Pressure transducers may be installed in both the large and small volume rails.
A method for selecting between a small volume rail and a large volume rail in a high pressure fuel system for a reciprocating internal combustion engine comprises the steps of initiating cranking of said internal combustion engine upon sensing key on, closing a valve in a fuel line between a high pressure fuel pump and the large volume rail, closing a valve in the fuel line between the large volume rail and the small volume rail, opening the valve in the fuel line between the large volume rail and the small volume rail after a predetermined time following cranking, and opening the valve in the fuel line between the high pressure fuel pump and the large volume rail after a predetermined time following cranking. The valve between the large volume rail and small volume rail is opened successively according to a schedule based on sensors measuring engine operating parameters including at least engine speed.
Alternatively, the opening of the valve in the fuel line between the large volume rail and the small volume rail may be determined based on pressure measured in the small volume rail. Furthermore, the valve located in between the large volume rail and small volume rail is opened variably such that the pressure in the small volume rail does not drop substantially below normal operating pressure. Similarly, the valve in the fuel line between the high pressure fuel pump and the large volume rail may be opened when a pressure transducer disposed in the large volume rail indicates that the pressure in the large volume rail is within a predetermined fraction of normal operating pressure.
The total internal volume of the low volume rail (including all valves, lines, etc.) should be proportional to the number of cylinders times volume per injection during engine start supplemented by a small margin. The purpose of the margin is to account for system variations such as the effect of ambient temperature on the hydraulic losses in the system.
Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification.